The rook occupies a central position. This is an active square, but it also leaves the rook open to an attack by the f-pawn.
13... e5f4 The sharpest continuation. 14. f1f4 This is the point of White's plan. The rook becomes active, the knight can go to d3, and we have a strange position on the board with White's central pawns still on their initial squares. 14... d5c4  15. b3c4  15... e7g6 